Greenhouse screens are used frequently for energy saving, shading and temperature control. One known type of greenhouse screens comprises a plurality of flexible strips of film material extending in parallel and which by means of a knitting, warp-knitting or weaving process and a yarn system are interconnected to form a continuous product, where the strips form a major part of the surface area of the product. Such a greenhouse screen is known for example through EP 0 109 951. Other examples of screens of this type are shown in FR 2 071 064, EP 1 342 824 and in WO 2008/091192.
The strips of flexible material can be of selected materials providing desired properties with respect to reflection and light and heat transmission.
JP 2000079671 discloses a polyester film useful as agricultural covering material. At least one side of the polyester film is covered with a coating layer dispersed with a photostabilizing agent.
WO 02/070253 and WO 02/0255 disclose multilayer films of bibenzol-modified polyester, the films being UV stable and may be used for outdoor purposes. They may contain a white pigment and/or a black pigment. The thickness of the films in the examples is about 150 μm.
The aim of protected cultivation in greenhouses is to modify the natural environment to increase yield, to improve product quality, to conserve resources, to extend production areas and crop cycles among others.
Supplementary lighting during the dark hours is used to increase the production of common greenhouse crops such as roses and tomatoes. Light pollution from greenhouses using supplementary lighting is a growing issue around the world. In the Netherlands, a country with an advanced cluster of greenhouses, this has become such a big problem for the society that legislation is in place to limit the amount of light that is allowed to escape the greenhouse during dark hours.
For some crops, for example chrysantimum and gerbera, the grower wants to be able to control the day length. This is necessary in order to maximize production and to get the right quality to the market at the right time.
The generally accepted method to prevent the light from escaping from or into a greenhouse, is to use a greenhouse screen. The screen limits the light that is allowed to escape or to enter to normally <1%. This is done by using opaque plastic strips woven or knitted together with a yarn to create a structure as described above. Normally, the yarn structure is very important to avoid excessive heat and humidity build-up in the greenhouse.
Screens in greenhouses can be a potential fire hazard, since a fire starting by for example an electrical failure in an assimilation lighting can be spread to the entire greenhouse by the screen causing huge economical damages.
For many crops, the rule of thumb is, 1% less light equals 1% less production. Using the assimilation lights together with a closed screen with good reflection increases light gain 4-5% at crop level. Hence, in practice screens with reflection >50% of visible light on at least one side is preferably used.
The bundle size is a very important feature of the screen. A big bundle of a single screen can cause losses of ˜3% of natural light, hence to maximize production the screen bundle size should be minimized. In black out applications, two layers of screens are often used on top of each other, further increasing the problems with bundle size.
The bundle size is controlled by the thickness of the opaque film that is used. In general, multilayer polyolefin films are the standard in the industry. This is due to their low cost, and the known technology to make them flame retardant and opaque with reflective surface(s). However, to be able to make the films sufficiently opaque and with at least one reflective side, the film thickness typically has to be at least 60-70 μm. This creates a relatively big screen bundle.